The instant invention relates to a new process and the corresponding plant for the transformation of dangerous waste containing chromium six (CR+6) into non dangerous wastes that meet the requirement of the CRETIB analysis.
The process includes the following steps: milling, water suspension, partial recovery of the chromium six, chemical treatment for the reduction of all the chromium six into chromium three through the action of a mixture of sulfurous anhydride gas with residual sulfuric acid from the manufacturing of liquid vaselines, basification. Finally, the resulting product, once meeting the requirements of the CRETIB analysis, can be used as raw material for the manufacturing of firebricks or for storage without any special requirement because it is now a non dangerous waste.
The abovementioned dangerous wastes are generated mainly in the preparation of sodium bichromate through alkaline fusion of chromium mineral under an oxidizing atmosphere in several types of furnaces depending on the process used, extracting in a second step, through lixiviation, a sodium chromate solution which is then transformed through acidification into sodium bichromate, obtaining as sub-products alumina or aluminum hydroxide contaminated with variable amounts of chromium six that, together with the final residues of the lixiviation are dangerous wastes. It is possible to recrystallize the sodium sulfate to purify it, to be used as raw material in the paper industry or in several chemical processes.
As mentioned above, the contaminated alumina and the lixiviation residue constitute dangerous wastes.
The lixiviation residue is constituted by insoluble chromates, iron salts, complex silicates including chromium in their composition, as well as chromium salts encapsulated by the silicates and insoluble salts. This makes their reduction and/or extraction difficult, and thus the instant invention is focused onto the elimination of said barriers and to obtain 100% efficacy in the reduction processes, verified for the fulfillment of the requirements of the CRETIB analysis. Hereinafter the documents of the state of the art related to the instant invention are presented in order to be included as references.
MX Patent Application No. 9300307 filed on Jan. 21, 1993 by Luigi Stoppani S. P. A. presents background information related to the art to which the instant invention belongs. The mentioned MX Patent Application presents a treatment at 450 Celsius degrees under a sulfur atmosphere with low oxygen contents, in a tubular furnace closed at its ends and with indirect heating. The advantage of the instant invention is that it does not require the use of a complicated installation and process as described in the abovementioned patent application. As mentioned above, prior milling and water suspension, a reducing mixture of sulfurous anhydride and sulfuric acid waste from vaseline manufacturing is used in reactors equipped with stirring devices without the need to heat or isolate the system from the atmosphere. It does not exert an influence on the reaction; it is a batch process, but presenting a continuous flow because at least three reactors are used sequentially. The total reduction is ensured through the criteria of CRETIB analysis, the resulting material is usable in the manufacture of bricks or firebricks and thus the natural mineral resources are used to their fullest extent. The process described in the instant invention and Stoppani's process are two very different processes. Stoppani's process is conducted in a closed furnace through solid-gas reaction, wherein the control of the reaction and its efficacy are not divulged clearly and cannot be precisely grasped from the wording of the patent document. In the process of the instant invention, the reaction is conducted in a liquid medium in a reactor, wherein extraction of the chromium to be reduced is conducted first and then the steps of chemical reaction, acidification and further reduction in a perfectly homogeneous phase are carried out, the complete stabilization being verifiable through the CRETIB analysis, which is not the case of the Stoppani's process. Moreover, in the process of the instant invention, the efficacy control is perfectly traceable and verifiable at any time during the process.
The object of the instant invention is to obtain the protection justified and validated by the detailed description mentioned hereinafter and by the technical-scientific development invested in the research exposed in the examples described, and against a possible protection granted to third parties, because the previously mentioned state of the art does not totally eliminate chromium six and confines it as a dangerous waste.